In maternofetal transfer of passive immunity, IgG is transferred across cells of fetal membranous tissues. Inasmuch as the first step in the process, the binding of IgG to receptors on the cell membrane, appears to be the factor which determines the overall specificity, an investigation will be made into the nature of the cell membrane receptors. It will be in three parts: the preparation of soluble IgG receptor substance, and studies of its binding characteristics and biochemical properties. Solubilization of receptor substance from rabbit fetal yolk sac membrane will be done by brief exposure to the action of proteolytic enzymes and/or nonionic detergents. Soluble receptor will be labeled with 125 I or 3H and isolated by affinity adsorption to Sepharose-IgG, by the Farr technique, or by selective precipitation with polyethylene glycol. Measurements will be made of the binding of receptor to IgG and values will be obtained for the association constant and index of heterogeneity for the system. Compositional analysis will be carried out to assess the chemical nature of the receptor and systematic degradation will be undertaken to establish the components of its structure that are essential for its binding function.